The Cancer in Pregnancy Forum

Archived Questions and Answers
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Question:
I am approx 5 weeks pregnant but 3.5 months ago, i had a whole body scan (with 5 milicuries of radioactive iodine) and a PET scan done as my doctor suspected that my thyroid cancer had returned. i had thyroid cancer over 20 years ago and it was removed with radioactive iodine treatment after the surgery.
i am concerned about the risks to my pregnancy and baby. could you please let me know what type of risk the whole body scan and PET scan pose as they were fairly recent? all research i have read suggested 12 months after radioactive iodine treatment though i believe those were in reference to cancer treatment vs. just diagnostic testing.
thanks!

Answer:

If you are a patient, consult your physician. The following should not replace the assessment and advice you have been receiving or may receive from your physician (cancer specialist, obstetrician or other healthcare provider). It is offered for your information only.

The maximum amount of radiation that is considered permissible for fetal exposure is 50 Milligray units. The fetal radiation dose following a single PET-scan is unlikely to be higher than this level.

The location of the PET-scan as well as the frequency will influence the amount of fetal dose of radiation, and it is important for the technician to calculate the amount of maternal and fetal dose prior to the procedure.Since the PET scan was performed about 10 weeks prior to conception, there was no fetal exposure to the radiation.

With respect to the radioactive iodine used, the half life is about 8 days, which is the time it takes for half of the initial dose to be eliminated. Generally it is agreed that by 5 half lives, almost all of the drug is eliminated. Also fetal thyroid doesn't begin absorption and incorporation of the iodide till the tenth week of gestation, thus exposure during early gestation shouldn't pose a high risk. The small dose used for diagnostic purposes as well as the 10 week lapsed time prior to your conception, minimizes if not eliminate any type of exposure to the fetus, and we don't suspect an increased risk to the baby in this situation.

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